1. Field of the Invention
The present invention relates to a method and apparatus for correcting drift during automated focused ion beam (FIB) processing.
2. Description of Related Art
Where a sample surface is irradiated with a focused ion beam (FIB) by an FIB system to micromachine the surface, if the beam irradiation time is prolonged, the sample stage on which the sample is placed and electronics drift with time. Consequently, it may be impossible to perform beam lithography accurately. Especially, as design rules of semiconductor devices have become more exacting, the positional accuracy required in microlithography has decreased below 100 nm. To realize such a high positional accuracy, the amount of drift of the beam irradiation position during the lithography process needs to be reduced by a factor of about 10, i.e., be less than 10 nm. Therefore, an intrinsic mark is put on the sample. Based on positional deviation of the mark, error in the beam irradiation position due to drift is corrected (see, for example, Japanese Patent Laid-Open No. H9-274879 (paragraphs 0009-0011; FIG. 1)).
Another technique for correcting the beam irradiation position is also known. In particular, a straight geometry is delineated in a direction parallel to a sample cross section to be created. During the micromachining, this straight geometry to be processed is referenced. Positions lying along a limited straight line extending at right angles to the straight geometry are measured. Drift of the beam irradiation position is corrected using the measured positions (see, for example, Japanese Patent Laid-Open No. 2003-331775 (paragraphs 0017-0031; FIG. 1)).
In the prior art FIB system of this kind, the sample stage is moved using an accurate pulsed motor or a mark for the stage. Accordingly, after determining the stage position, micromachining is started, and the position is corrected. That is, correction of drift is made after start of the micromachining.
The present situation is that where automated micromachining involving movement of the sample stage is carried out at the ion beam position by the aforementioned prior art technique, movement of the stage is corrected using some mark. The FIB system is intended to automatically micromachine the sample. Because there are plural beam irradiation positions at which processing is performed, it has been difficult to reproduce the stage position accurately because of the mechanical accuracy. Consequently, a correction has been made by movement of the stage with enhanced mechanical accuracy of the stage or an intrinsic mark on the stage. In this way, the stage position has been reproduced.
Furthermore, where the method consisting of reading a signal reflected from a sample when it is irradiated with a beam and correcting the sample position is implemented, powder flying off the processed region may adhere to the reference image region or the processing beam may scatter within the processed region. Therefore, the resulting image becomes totally white or black. This makes it impossible to precisely read the image in the reference image region. Consequently, there arises the problem that the processing accuracy deteriorates.